WASHINGTON (CNN) -- Federal safety officials said Thursday they have identified the origin of the battery fire on a Boeing 787 Dreamliner last month, and are turning their microscopes on an aircraft approval process in which the airplane builder evidently greatly underestimated the chances of battery failure.
Boeing had estimated a "smoke" event would occur "less than once in 10 million flight hours" with the Dreamliner's novel lithium-ion batteries, National Transportation Safety Board chairwoman Deborah Hersman said. But after fewer than 100,000 hours of actual flight, two batteries failed, one culminating in a fire.
Further, Boeing's indications that heat damage in one battery cell would not harm adjacent cells proved false, Hersman said.
"The assumptions used to certify the battery must be reconsidered," Hersman said.
Hersman's statements cast doubt not only on the safety of the battery technology, but on the government's certification process for approving technology. It also appeared to dispel any hopes for a quick resolution to the problem, which has grounded all 50 Dreamliner aircraft globally since January 16th.
The NTSB plans to release an interim report of its findings within 30 days. The Federal Aviation Administration -- the ultimate arbiter of when the plane can resume flying -- has declined to predict when the 787 will return to the sky.
Thursday's briefing, incidentally, occurred as Boeing flew a Dreamliner for a one-time, special flight from Fort Worth, Texas, where it was being painted, to Boeing facilities in Everett, Washington.
The FAA approved the flight under the condition that the plane would only have essential crew members on board, that they continuously monitor the battery and that they land immediately if they detected an anomaly. The plane landed without incident; a Boeing fire truck awaiting its arrival. The FAA said Thursday that it has approved Boeing to conduct test flights with 787s to help gather more data.
In the mishap a month ago today, a battery pack caught fire aboard an empty Japan Airlines Dreamliner at Boston's Logan International Airport. Less than a week later, an All Nippon Airways Dreamliner bound for Hong Kong had to make an emergency landing after irregular battery activity and smoke in the forward electrical compartment were detected.
Hersman said the flight data recorder in the Boston incident showed the battery underwent an unexplained drop in voltage from 32 volts to 28 immediately before the incident, as the plane was being serviced on the tarmac. The voltage drop was consistent with the charge of a single cell on the eight-cell battery, she said.
Hersman said investigators believe the problem originated in cell six, which shows multiple signs of a short circuit -- an unintended path of electricity. The short circuit resulted in a thermal runaway -- a chemical chain reaction -- in cell six, which spread to adjacent cells.
"Charred battery components indicated that the temperature inside the battery case exceeded 500 degrees," Hersman said.
But investigators still don't know what caused cell six to short-circuit in the first place.
They have ruled out two possibilities -- mechanical "impact" damage, like that caused by being dropped, or short-circuiting outside the battery.
But several other possibilities are being explored, including contamination or defect during manufacturing, flaws in the design or construction of the battery, and problems with battery charging. That final possibility -- battery charging -- leaves open the possibility that the problem could reside outside the battery itself.
Boeing, in a statement issued Thursday, said, "We are working collaboratively to address questions about our testing and compliance with certification standards, and we will not hesitate to make changes that lead to improved testing processes and products."
When the FAA approved the use of lithium-ion batteries on the Dreamliner, it imposed nine "special conditions" to prevent or mitigate problems. One condition was that the battery design "preclude the occurrence" of thermal runaways. Another is that the plane have a system to control the charging rate of the battery to prevent over- or undercharging. Another: that there be a warning system for battery failures.
In a joint statement Thursday, Transportation Secretary Ray LaHood and FAA Administrator Michael Huerta reiterated that they are determined to fix the problem.
"Based on what information our experts find, the FAA will take any action necessary to further ensure safety. We must finish this work before reaching conclusions about what changes or improvements the FAA should make going forward," the statement read.
"The FAA is focused on the review and activities to understand the root cause. Once the review is complete, the FAA will make any analysis and conclusions public."